The subtraction of optical images has recently received growing interest due to the various disciplines interested in the extraction of the difference information between two images. Among the fields which may benefit from a reliable and easily interpretable subtraction method one finds earth resource studies, meteorology, area surveillance, medicine, aerial photography, microelectronics mask testing, etc.
The subtraction operation has been performed optically in the past, but most approaches (See Reference 1 of the References listed in Appendix II hereof) required several operations to be done sequentially, and very often an intermediate record had to be generated. This was sometimes a hologram on which information from the two images was properly recorded, (See References 2-6) and some other times a composite coded image (See References 7-13) from which the subtraction was later extracted by spatial filtering. Another approach has been the positive-negative superposition. See Reference 1.
In this latter technique two images to be subtracted are supplied to projection cathode ray tubes and one is inverted from positive to negative electrically by one of the tubes. The outputs are then optically superposed.
The techniques described in the literature do not fulfill all the requirements that a practical subtraction system should provide: large dynamic range, good signal-to-noise ratio (elimination of the common identical information), real time operation, speckle free image (utilization of spatially incoherent light).
It is an object of this invention to provide a system which overcomes these problems and does provide such a practical optical subtraction system.